THE WARM FRONT 27 
ee 
if carried far enough, will eventually 
lead to voluntary convection of the 
warm air, and hence shower type 
precipitation. In our Tropical Gulf 
air masses this is normally the case. 
One does not always find a continuous 
cloud layer above the warm front 
surface as indicated in figure 5. 
Rather, one often finds two or more 
layers of clouds separated by ¢ccm- 
paratively clear spaces. This does 
not invalidate the frontal idea, but 
merely indicates that the air is 
ascending the front in layers. Since 
the warm current is generally vb- 
served to be stratified in its tempera- 
ture and moisture distribution, 1t is 
probable that, as the successive 
layers ascend, condensation appears 
at some levels sooner than at others. 
The cloud layer, in the event of the 
release of marked instability, is apt 
to be thick. 
The structure and trajectory of 
the underlying cold air current, 
while not particularly important for 
the production of precipitation, is 
nevertheless significant for the types 
of clouds which are observed. Where 
the cold current has passed over a 
water surface, there will generally be 
found a layer of Stcu cloud, which 
obscures from view the upper layer 
of clouds associated with the warm 
front. The elevation of these lower 
clouds is rarely over 500 m, their 
thickness seldom appreciable, and the 
precipitation from them, if any, is 
only mist, or at most, drizzle. Occa- 
sionally, at intermediate levels within 
the cold air, there is found a layer 
of Acu clouds. These seem to be 
associated with surfaces of subsid- 
ence, to be discussed in a later article. 
In the cold air through which rain 
is falling, conditions are becoming 
increasingly favorable for the forma- 
tion of clouds, for, in the first piace, 
the moisture content is increased by 
evaporating rain and secondly, in the 
cold air adjacent to the surface of 
discontinuity, the lapse rate tends to 
steepen. 
In the lowest layers of the cold 
air, perhaps 100 m above the surface 
of the earth, Frst clouds (scud) may 
be formed by turbulence in this 
almost saturated lowest layer of air. 
The identification of warm fronts 
on the surface weather map is fre- 
quently very difficult. The wind dis- 
continuity may not be pronounced— 
and to complicate matters further the 
transition of temperature may often 
appear relatively gradual. This dif- 
fuse distribution of surface elements 
across a warm front is explained by 
the small angle of slope of the dis- 
continuity surface. At high levels 
it is probable that there is little mix- 
ing of the warm and cold air, but in 
the turbulent layer lying within about 
600 m of the surface, if the dis- 
continuity surface is not far from 
the ground, there must be some con- 
siderable intermingling of the warm 
air with the lower cold air. Thus, 
the temperature and moisture con- 
tent of the air some distance ahead 
of the warm front gradually increase. 
A good rule to remember in this con- 
nection is that the gradual changes 
which occur in the transition zone are 
entirely within the cold air. Thus in 
the warm air current the meteoro- 
logical elements, particularly temper- 
ature and moisture, should remain 
comparatively constant. 
Upper air soundings offer the best 
aid to the identification of warm 
fronts. As pointed out in Article IV 
of this series, a sounding through a 
warm front appears on the Rossby 
diagram as a wedge-shaped curve 
with a maximum water vapor con- 
tent in mid-air (fig. 5, in Article IV). 
The base of the warm current is gen- 
erally considered as the point of the 
